1. Field of the Invention
The invention relates generally to the field of laser capture microdissection (LCM). More particularly, the invention relates to inverted microscopes that include specialized apparatus for performing LCM. Specifically, a preferred implementation of the invention relates to an inverted microscope that includes a cap handling subsystem, an illumination/laser optical subsystem, a vacuum chuck subsystem, and a manual joystick subsystem. The invention thus relates to inverted microscopes of the type that can be termed laser capture microdissection inverted microscopes.
2. Discussion of the Related Art
Diseases such as cancer have long been identified by examining tissue biopsies to identify unusual cells. The problem has been that there has been no satisfactory prior-art method to extract the cells of interest from the surrounding tissue. Currently, investigators must attempt to manually extract, or microdissect, cells of interest either by attempting to mechanically isolate them with a manual tool or through a convoluted process of isolating and culturing the cells. Most investigators consider both approaches to be tedious, time-consuming, and inefficient.
A new technique has been developed which can extract a small cluster of cells from a tissue sample in a matter of seconds. The technique is called laser capture microdissection (LCM). Laser capture microdissection is a one-step technique which integrates a standard laboratory microscope with a low-energy laser and a transparent ethylene vinyl acetate polymer thermoplastic film such as is used for the plastic seal in food product packaging.
In laser capture microdissection, the operator looks through a microscope at a tissue biopsy section mounted on a standard glass histopathology slide, which typically contains groups of different types of cells. A thermoplastic film is placed over and in contact with the tissue biopsy section. Upon identifying a group of cells of interest within the tissue section, the operator centers them in a target area of the microscope field and then generates a pulse from a laser such as a carbon dioxide laser having an intensity of about 50 milliwatts (mW) and a pulse duration of between about 50 to about 500 milliseconds (mS). The laser pulse causes localized heating of the plastic film as it passes through it, imparting to it an adhesive property. The cells then stick to the localized adhesive area of the plastic tape directly above them, whereupon the cells are immediately extracted and ready for analysis. Because of the small diameter of the laser beam, extremely small cell clusters may be microdissected from a tissue section.
By taking only these target cells directly from the tissue sample, scientists can immediately analyze the gene and enzyme activity of the target cells using other research tools. Such procedures as polymerase chain reaction amplification of DNA and RNA, and enzyme recovery from the tissue sample have been demonstrated. No limitations have been reported in the ability to amplify DNA or RNA from tumor cells extracted with laser capture microdissection.
Laser capture microdissection has successfully extracted cells in all tissues in which it has been tested. These include kidney glomeruli, in situ breast carcinoma, a typical ductal hyperplasia of the breast, prostatic interepithielial neoplasia, and lymphoid follicles. The direct access to cells provided by laser capture microdissection will likely lead to a revolution in the understanding of the molecular basis of cancer and other diseases, helping to lay the groundwork for earlier and more precise disease detection.
Another likely role for the technique is in recording the patterns of gene expression in various cell types, an emerging issue in medical research. For instance, the National Cancer Institute's Cancer Genome Anatomy Project (CGAP) is attempting to define the patterns of gene expression in normal, precancerous, and malignant cells. In projects such as CGAP, laser capture microdissection is a valuable tool for procuring pure cell samples from tissue samples.
The LCM technique is generally described in the recently published article: Laser Capture Microdissection, Science, Volume 274, Number 5289, Issue 8, pp 998–1001, published in 1996, the entire contents of which are incorporated herein by reference. The purpose of the LCM technique is to provide a simple method for the procurement of selected human cells from a heterogeneous population contained on a typical histopathology biopsy slide.
A typical tissue biopsy sample consists of a 5 to 10 micron slice of tissue that is placed on a glass microscope slide using techniques well known in the field of pathology. This tissue slice is a cross section of the body organ that is being studied. The tissue consists of a variety of different types of cells. Often a pathologist desires to remove only a small portion of the tissue for further analysis.
LCM employs a thermoplastic transfer film that is placed on top of the tissue sample. This film is manufactured containing organic dyes that are chosen to selectively absorb in the near infrared region of the spectrum overlapping the emission region of common AlGaAs laser diodes. When the film is exposed to the focused laser beam the exposed region is heated by the laser and melts, adhering to the tissue in the region that was exposed. The film is then lifted from the tissue and the selected portion of the tissue is removed with the film.
Thermoplastic transfer films such as a 100 micron thick ethyl vinyl acetate (EVA) film available from Electroseal Corporation of Pompton Lakes, N.J. (type E540) have been used in LCM applications. The film is chosen to have a low melting point of about 90° C.
The thermoplastic EVA films used in LCM techniques have been doped with dyes, such as an infrared napthalocyanine dye, available from Aldrich Chemical Company (dye number 43296-2 or 39317-7). These dyes have a strong absorption in the 800 nm region, a wavelength region that overlaps with laser emitters used to selectively melt the film. The dye is mixed with the melted bulk plastic at an elevated temperature. The dyed plastic is then manufactured into a film using standard film manufacturing techniques. The dye concentration in the plastic is about 0.001 M.
While the films employed in LCM applications have proved satisfactory for the task, they have several drawbacks. The optical absorption of a dye impregnated film is a function of its thickness. This property of the film may be in conflict with a desire to select film thickness for other reasons.
The organic dyes which are used to alter the absorption characteristics of the films may have detrimental photochemistry effects in some cases. This could result in contamination of LCM samples. In addition, the organic dyes employed to date are sensitive to the wavelength of the incident laser light and thus the film must be matched to the laser employed.